1. Field of the Invention
This invention relates to electrical apparatus for the measuring and testing of signal circuits, and it relates particularly to an intrinsic safety barrier which connects to signal lines for preventing excessive voltages or currents thereon.
2. Description of the Prior Art
Instrumentation within processing facilities, such as chemical and oil refining complexes, are now designed to include various devices for providing intrinsically safe installations. Severe engineering and operating parameters are encountered in the design of such systems. The various systems must permit advanced technology signal transmission and processing capabilities while yet preventing the passage of excess voltages and current which could cause ignition problems within flammable or explosive atmospheres. In addition, the Occupational Safety and Health Act (OSHA) has made mandatory the use of intrinsically safe installations for the protection of property and operating personnel. Generally, the components of the intrinsically safe installations must be tested and certified safe in hazardous environments. For example, the hazardous area to be protected can be the operational portions of an oil refinery contaminated with various gases creating flammable or explosive atmospheres. Instrumentation may be placed within a remote control room, but an intrinsically safe installation requires some type of barrier to be placed between the control room (safe area) and the operating facility (hazardous area) so that no excessive voltages or currents can cause an explosion outside of the control room.
The design of the instrument system must be such that the instrumentation circuit in the hazardous area cannot release sufficient energy under any combination of conditions to ignite the explosive atmosphere. Irrespective of malfunctions or improper installation or servicing, the instrument system must failsafe, and in no event can it possibly be so mismanaged as to create an explosion. Most intrinsically safe systems use circuit elements which place absolute limits on currents and voltages available at any instant in a hazardous area. Various circuit components prevent accumulation and release of sufficient energy from capacitance and inductance effects so as to cause an explosion. The low energy signal levels of solid state instrumentation devices make difficult the design of an intrinsically safe instrumentation system for them. For example, reference may be taken to U.S. Pat. No. 3,717,566, which describes an industrial model of an automatic corrosion ratemeter for determining the corrosion occurring at remotely placed probes. The instrumentation package is placed within a control room and conductors extend for some distance to various probes placed at sensing locations within the processing streams. For example, the probes may be located at distances of up to one mile from the instrumentation package. The signal levels upon the various conductors are in the range of not over several hundred millivolts and several milliamperes. These signal levels are very low, and they are not close to the magnitudes that can create the ignition of an explosive environment unless ignited by malfunctioning instrumentation. For this purpose, it is required to design a barrier which may be interposed between the instrumentation and the conductors which extend to the remotely positioned probes without deterioration of signal levels.
Prior art barriers have been known which employ voltage limiting zener diodes connected redundantly, e.g., in parallel, with series resistors for current limiting and test purposes. In addition, various unidirectional devices such as diodes can be employed for purposes of limiting both voltages and currents. However, these systems all had undesired effects in that they effected the small signal levels encountered in the normal operation of highly sensitive instruments such as the mentioned corrosion ratemeter. For example, the zener diode has some conduction even at small potential and current levels other than where its limiting action is desired. Also, most barriers presently available are not bipolar and can handle only one polarity of potential with respect to ground potential.
The above deficiences in known systems led to the development of a redundant intrinsic safety barrier of the present invention. This safety barrier employs a plurality of elements such that, within predetermined limits, voltages and current excesses may be reduced to levels at circuit common by operation of its various elements. Should voltages or currents above the predetermined operating levels be encountered, these elements self-latch into a condition causing a series-connected fuse to open and thereby completely provide a failsafe operation of the barrier. Naturally, the present safety barrier does not influence the normal operation at low signal levels of highly sensitive instrumentation such as the mentioned corrosion ratemeter. However, should an unsafe operating condition through excess voltages or currents occur, the device will either automatically limit them or cause an automatic fuse failure to open the signal circuits so that there is no possibility of the appearance of such excess voltage and current appearing in a hazardous location where a probe is located.